The PR effect in LN is an action of several cascaded processes that induce a refractive index variation of the material in presence of light illumination [1]}. Due to the broken inversion symmetry of its crystal structure, when the LN is illuminated, a photocurrent is generated along crystalline z-direction through bulk photovoltaic effect. The migrated photo-induced charges are then trapped in defects of crystal, building up a space-charge field opposite to the direction of photocurrent. The electric field subsequently modulates the refractive index of the LN crystal through the Pockels effect. Specific to fabricated LNOI microcavities discussed here, the drop of refractive index results in a blue shift of resonance frequency with a relaxation time ranging from tens of milliseconds to several seconds at room temperature [2]}, [3]}.
